Electro-optic displays with reduced remnant voltage

ABSTRACT

The invention provides materials and methods (including driving methods) for reducing the effects of remnant voltages in electro-optic displays.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending application Ser.No. 10/879,335, filed Jun. 29, 2004 (Publication No. 2005/0024353),which itself is a continuation-in-part of copending application Ser. No.10/814,205, filed Mar. 31, 2004 (Publication No. 2005/0001812, now U.S.Pat. No. 7,119,772), which itself is a continuation-in-part of copendingapplication Ser. No. 10/065,795, filed Nov. 20, 2002 (Publication No.2003/0137521, now U.S. Pat. No. 7,012,600). The aforementionedapplication Ser. No. 10/879,335 claims benefit of Application Ser. No.60/481,040, filed Jun. 30, 2003, of Application Ser. No. 60/481,053,filed Jul. 2, 2003, and of Application Ser. No. 60/481,405, filed Sep.22, 2003. The aforementioned application Ser. No. 10/814,205 claimsbenefit of Application Ser. No. 60/320,070, filed Mar. 31, 2003, ofApplication Ser. No. 60/320,207, filed May 5, 2003, of Application Ser.No. 60/481,669, filed Nov. 19, 2003, of Application Ser. No. 60/481,675,filed Nov. 20, 2003 and of Application Ser. No. 60/557,094, filed Mar.26, 2004. The aforementioned application Ser. No. 10/065,795 claimsbenefit of Application Ser. No. 60/319,007, filed Nov. 20, 2001, ofApplication Ser. No. 60/319,010, filed Nov. 21, 2001, of ApplicationSer. No. 60/319,034, filed Dec. 18, 2001, of Application Ser. No.60/319,037, filed Dec. 20, 2001, and of Application Ser. No. 60/319,040,filed Dec. 21, 2001.

This application also claims benefit of Application Ser. No. 60/481,711,filed Nov. 26, 2003, and of Application Ser. No. 60/481,713, filed Nov.26, 2003.

This application is related to copending application Ser. No.10/249,973, filed May 23, 2003 (now U.S. Pat. No. 7,193,625), which is acontinuation-in-part of the aforementioned application Ser. No.10/065,795, application Ser. No. 10/249,973 also claims benefit ofApplication Ser. No. 60/319,315, filed Jun. 13, 2002 and of ApplicationSer. No. 60/319,321, filed Jun. 18, 2002. This application is alsorelated to copending application Ser. No. 10/063,236, filed Apr. 2, 2002(Publication No. 2002/0180687, now U.S. Pat. No. 7,170,670).

The entire contents of the aforementioned applications are hereinincorporated by reference. The entire contents of all United StatesPatents and published and copending Applications mentioned below arealso herein incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to electro-optic displays with reduced remnantvoltage, and to methods for reducing remnant voltage in electro-opticdisplays. The term “remnant voltage” is used herein to refer to apersistent or decaying electric field that remains in certainelectro-optic displays after an addressing pulse (a voltage pulse usedto change the optical state of the electro-optic medium) is terminated.It has been found that such remnant voltages can lead to undesirableeffects on the images displayed on electro-optic displays; inparticular, remnant voltages can lead to so-called “ghosting” phenomena,in which, after the display has been rewritten, traces of the previousimage are still visible. The present invention is especially, though notexclusively, intended for use in electrophoretic displays.

Electro-optic displays comprise a layer of electro-optic material, aterm which is used herein in its conventional meaning in the imaging artto refer to a material having first and second display states differingin at least one optical property, the material being changed from itsfirst to its second display state by application of an electric field tothe material. Although the optical property is typically colorperceptible to the human eye, it may be another optical property, suchas optical transmission, reflectance, luminescence or, in the case ofdisplays intended for machine reading, pseudo-color in the sense of achange in reflectance of electromagnetic wavelengths outside the visiblerange.

In the displays of the present invention, the electro-optic medium willtypically be a solid (such displays may hereinafter for convenience bereferred to as “solid electro-optic displays”), in the sense that theelectro-optic medium has solid external surfaces, although the mediummay, and often does, have internal liquid- or gas-filled spaces. Thus,the term “solid electro-optic displays” includes encapsulatedelectrophoretic displays, encapsulated liquid crystal displays, andother types of displays discussed below.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the patentsand published applications referred to below describe electrophoreticdisplays in which the extreme states are white and deep blue, so that anintermediate “gray state” would actually be pale blue. Indeed, asalready mentioned the transition between the two extreme states may notbe a color change at all.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin published U.S. Patent Application No. 2002/0180687 that someparticle-based electrophoretic displays capable of gray scale are stablenot only in their extreme black and white states but also in theirintermediate gray states, and the same is true of some other types ofelectro-optic displays. This type of display is properly called“multi-stable” rather than bistable, although for convenience the term“bistable” may be used herein to cover both bistable and multi-stabledisplays.

The term “impulse” is used herein in its conventional meaning in theimaging art of the integral of voltage with respect to time. However,some bistable electro-optic media act as charge transducers, and withsuch media an alternative definition of impulse, namely the integral ofcurrent over time (which is equal to the total charge applied) may beused. The appropriate definition of impulse should be used, depending onwhether the medium acts as a voltage-time impulse transducer or a chargeimpulse transducer.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedby applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. No. 6,301,038, International Application Publication No. WO01/27690, and in U.S. Patent Application 2003/0214695. This type ofmedium is also typically bistable.

Another type of electro-optic display, which has been the subject ofintense research and development for a number of years, is theparticle-based electrophoretic display, in which a plurality of chargedparticles move through a suspending fluid under the influence of anelectric field. Electrophoretic displays can have attributes of goodbrightness and contrast, wide viewing angles, state bistability, and lowpower consumption when compared with liquid crystal displays.Nevertheless, problems with the long-term image quality of thesedisplays have prevented their widespread usage. For example, particlesthat make up electrophoretic displays tend to settle, resulting ininadequate service-life for these displays.

As noted above, electrophoretic media require the presence of asuspending fluid. In most prior art electrophoretic media, thissuspending fluid is a liquid, but electrophoretic media can be producedusing gaseous suspending fluids; see, for example, Kitamura, T., et al.,“Electrical toner movement for electronic paper-like display”, IDWJapan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., “Toner displayusing insulative particles charged triboelectrically”, IDW Japan, 2001,Paper AMD4-4). See also European Patent Applications 1,429,178;1,462,847; and 1,482,354; and International Applications WO 2004/090626;WO 2004/079442; WO 2004/077140; WO 2004/059379; WO 2004/055586; WO2004/008239; WO 2004/006006; WO 2004/001498; WO 03/091799; and WO03/088495. Such gas-based electrophoretic media appear to be susceptibleto the same types of problems due to particle settling as liquid-basedelectrophoretic media, when the media are used in an orientation whichpermits such settling, for example in a sign where the medium isdisposed in a vertical plane. Indeed, particle settling appears to be amore serious problem in gas-based electrophoretic media than inliquid-based ones, since the lower viscosity of gaseous suspendingfluids as compared with liquid ones allows more rapid settling of theelectrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspending medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545;6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333;6,704,133; 6,710,540; 6,721,083; 6,727,881; 6,738,050; 6,750,473;6,753,999; 6,816,147; 6,819,471; and 6,822,782; and U.S. PatentApplications Publication Nos. 2002/0019081; 2002/0060321;[[2002/0060321;]]2002/0063661; 2002/0090980; 2002/0113770; 2002/0130832;2002/0131147; 2002/0171910; 2002/0180687; 2002/0180688; 2003/0011560;2003/0020844; 2003/0025855; 2003/0053189; 2003/0102858; 2003/0132908;2003/0137521; 2003/0137717; 2003/0151702; 2003/0214695; 2003/0214697;2003/0222315; 2004/0008398; 2004/0012839; 2004/0014265; 2004/0027327;2004/0075634; 2004/0094422; 2004/0105036; 2004/0112750; and2004/0119681; and International Applications Publication Nos. WO99/67678; WO 00/05704; WO 00/38000; WO 00/38001; WO00/36560; WO00/67110; WO 00/67327; WO 01/07961; WO 01/08241; WO 03/107,315; WO2004/023195; WO 2004/049045; WO 2004/059378; WO 2004/088002; WO2004/088395; and WO 2004/090857.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned 2002/0131147. Accordingly, for purposes of thepresent application, such polymer-dispersed electrophoretic media areregarded as sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the suspending fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, International Application Publication No. WO 02/01281, andpublished U.S. Application No. 2002/0075556, both assigned to SipixImaging, Inc.

Many of the aforementioned E Ink and MIT patents and applications alsocontemplate microcell electrophoretic displays and polymer-dispersedelectrophoretic displays. The term “encapsulated electrophoreticdisplays” can refer to all such display types, which may also bedescribed collectively as “microcavity electrophoretic displays” togeneralize across the morphology of the walls.

Another type of electro-optic display is an electro-wetting displaydeveloped by Philips and described in Hayes, R. A., et al., “Video-SpeedElectronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003).It is shown in copending application Ser. No. 10/711,802, filed Oct. 6,2004, that such electro-wetting displays can be made bistable.

Other types of electro-optic materials may also be used in the presentinvention. Of particular interest, bistable ferroelectric liquid crystaldisplays (FLC's) are known in the art and have exhibited remnant voltagebehavior.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, theaforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat.Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856.Dielectrophoretic displays, which are similar to electrophoreticdisplays but rely upon variations in electric field strength, canoperate in a similar mode; see U.S. Pat. No. 4,418,346. Other types ofelectro-optic displays may also be capable of operating in shutter mode.

An encapsulated or microcell electrophoretic display typically does notsuffer from the clustering and settling failure mode of traditionalelectrophoretic devices and provides further advantages, such as theability to print or coat the display on a wide variety of flexible andrigid substrates. (Use of the word “printing” is intended to include allforms of printing and coating, including, but without limitation:pre-metered coatings such as patch die coating, slot or extrusioncoating, slide or cascade coating, curtain coating; roll coating such asknife over roll coating, forward and reverse roll coating; gravurecoating; dip coating; spray coating; meniscus coating; spin coating;brush coating; air knife coating; silk screen printing processes;electrostatic printing processes; thermal printing processes; inkjetprinting processes; electrophoretic deposition; and other similartechniques.) Thus, the resulting display can be flexible. Further,because the display medium can be printed (using a variety of methods),the display itself can be made inexpensively.

The bistable or multi-stable behavior of particle-based electrophoreticdisplays, and other electro-optic displays displaying similar behavior(such displays may hereinafter for convenience be referred to as“impulse driven displays”), is in marked contrast to that ofconventional liquid crystal (“LC”) displays. Twisted nematic liquidcrystals act are not bi- or multi-stable but act as voltage transducers,so that applying a given electric field to a pixel of such a displayproduces a specific gray level at the pixel, regardless of the graylevel previously present at the pixel. Furthermore, LC displays are onlydriven in one direction (from non-transmissive or “dark” to transmissiveor “light”), the reverse transition from a lighter state to a darker onebeing effected by reducing or eliminating the electric field. Finally,the gray level of a pixel of an LC display is not sensitive to thepolarity of the electric field, only to its magnitude, and indeed fortechnical reasons commercial LC displays usually reverse the polarity ofthe driving field at frequent intervals. In contrast, bistableelectro-optic displays act, to a first approximation, as impulsetransducers, so that the final state of a pixel depends not only uponthe electric field applied and the time for which this field is applied,but also upon the state of the pixel prior to the application of theelectric field.

Also, to obtain a high-resolution display, individual pixels of adisplay must be addressable without interference from adjacent pixels.One way to achieve this objective is to provide an array of non-linearelements, such as transistors or diodes, with at least one non-linearelement associated with each pixel, to produce an “active matrix”display. An addressing or pixel electrode, which addresses one pixel, isconnected to an appropriate voltage source through the associatednon-linear element. Typically, when the non-linear element is atransistor, the pixel electrode is connected to the drain of thetransistor, and this arrangement will be assumed in the followingdescription, although it is essentially arbitrary and the pixelelectrode could be connected to the source of the transistor.Conventionally, in high resolution arrays, the pixels are arranged in atwo-dimensional array of rows and columns, such that any specific pixelis uniquely defined by the intersection of one specified row and onespecified column. The sources of all the transistors in each column areconnected to a single column electrode, while the gates of all thetransistors in each row are connected to a single row electrode; againthe assignment of sources to rows and gates to columns is conventionalbut essentially arbitrary, and could be reversed if desired. The rowelectrodes are connected to a row driver, which essentially ensures thatat any given moment only one row is selected, i.e., that there isapplied to the selected row electrode a voltage such as to ensure thatall the transistors in the selected row are conductive, while there isapplied to all other rows a voltage such as to ensure that all thetransistors in these non-selected rows remain non-conductive. The columnelectrodes are connected to column drivers, which place upon the variouscolumn electrodes voltages selected to drive the pixels in the selectedrow to their desired optical states. (The aforementioned voltages arerelative to a common front electrode which is conventionally provided onthe opposed side of the electro-optic medium from the non-linear arrayand extends across the whole display.) After a pre-selected intervalknown as the “line address time” the selected row is deselected, thenext row is selected, and the voltages on the column drivers are changedto that the next line of the display is written. This process isrepeated so that the entire display is written in a row-by-row manner.

The aforementioned 2003/0137521 describes how a direct current (DC)imbalanced waveform can result in a remnant voltage being created, thisremnant voltage being ascertainable by measuring the open-circuitelectrochemical potential of a display pixel.

For reasons explained at length in the aforementioned copendingapplications, when driving an electro-optic display it is desirable touse a drive scheme that is DC balanced, i.e., on which has the propertythat, for any sequence of optical states, the integral of the appliedvoltage is zero whenever the final optical state matches the initialoptical state. This guarantees that the net DC imbalance experienced bythe electro-optic layer is bounded by a known value. For example, a 15V, 300 ms pulse may be used to drive an electro-optic layer from thewhite to the black state. After this transition, the imaging layer hasexperienced 4.5 V-s of DC-imbalanced impulse. To drive the film back towhite, if a −15 V, 300 ms pulse is used, then the imaging layer is DCbalanced across the series of transitions from white to black and backto white.

It has now been found that remnant voltage is a more general phenomenonin electrophoretic and other impulse-driven electro-optic displays, bothin causes and effects. It has also been found that DC imbalances causelong-term lifetime degradation of electrophoretic displays.

Remnant voltage has been measured in electrophoretic displays bystarting with a sample that has not been switched for a long period oftime (e.g. hours or days). A voltmeter is applied across the open pixelcircuit and a “Base Voltage” reading is measured. An electric field isthen applied to the pixel, for example a switching waveform. Immediatelyafter the waveform ends, the voltmeter is used to measure theopen-circuit potential over a series of periods, and the differencebetween the measured reading and the original Base Voltage is regardedas the “remnant voltage”.

The remnant voltage decays in a complex manner which may be looselyapproximated mathematically as a sum of exponentials. In typicalexperiments, 15 V was applied across the electro-optic medium forapproximately 1 second. Immediately after the end of this voltage pulse,a remnant voltage of between +3 V and −3 V was measured; 1 second latera remnant voltage of between +1 V and −1 V was measured; ten minuteslater the remnant voltage was near zero (relative to the original BaseVoltage).

The term “remnant voltage” is sometimes used herein as a term ofconvenience referring to an overall phenomenon. However the basis forthe switching behavior of impulse-driven electro-optic displays is theapplication of a voltage impulse (the integral of voltage with respectto time) across the electro-optic medium. As shown in FIG. 1 of theaccompanying drawings, which is a typical graph of remnant voltageagainst time, remnant voltage reaches a peak value, designated 102,immediately after the application of a driving pulse (the time scale inFIG. 1 is essentially arbitrary), and thereafter decays substantiallyexponentially, as indicated by curve 104 in FIG. 1. The persistence ofthe remnant voltage for a significant time period applies a “remnantimpulse” represented by the area 106 under curve 104, to theelectro-optic medium, and strictly speaking it is this remnant impulse,rather than the remnant voltage, that is responsible for the effects onthe optical states of electro-optic displays normally considered ascaused by remnant voltage.

In theory the effect of remnant voltage should correspond directly toremnant impulse. In practice, however, the impulse switching model canlose accuracy at low voltages. Some electro-optic media, includingpreferred electrophoretic media used in experiments described herein,have a small threshold, such that a remnant voltage of about 1 V doesnot cause a noticeable change in the optical state of the medium after adrive pulse ends. Thus, two equivalent remnant impulses may differ inactual consequences, and it may be helpful to increase the threshold ofthe electro-optic medium to reduce the effect of remnant voltage. E Inkhas produced electrophoretic media having a “small threshold” adequateto prevent remnant voltage experienced in typical use from immediatelychanging the display image after a drive pulse ends. If the threshold isinadequate or if the remnant voltage is too high, the display maypresent a kickback/self-erasing or self-improving phenomenon.

Even when remnant voltages are below a small threshold, they do have aserious effect on image switching if they still persist when the nextimage update occurs. For example, suppose that during an image update ofan electrophoretic display a +/−15 V drive voltage is applied to movethe electrophoretic particles. If a +1 V remnant voltage persists from aprior update, the drive voltage would effectively be shifted from +15V/−15 V to +16 V/−14 V. As a result, the pixel would be biased towardthe dark or white state, depending on whether it has a positive ornegative remnant voltage. Furthermore, this effect varies with elapsedtime due to the decay rate of the remnant voltage. The electro-opticmaterial in a pixel switched to white using a 15 V, 300 ms drive pulseimmediately after a previous image update may actually experience awaveform closer to 16 V for 300 ms, whereas the material in a pixelswitched to white one minute later using the exact same drive pulse (15V, 300 ms) may actually experience a waveform closer to 15.2 V for 300ms. Consequently the pixels may show noticeably different shades ofwhite.

If the remnant voltage field has been created across multiple pixels bya prior image (say a dark line on a white background) then the remnantvoltages may also be arrayed across the display in a similar pattern. Inpractical terms then, the most noticeably effect of remnant voltage ondisplay performance is ghosting. This problem is in addition to theproblem previously noted, namely that DC imbalance (e.g. 16 V/14 Vinstead of 15 V/15 V) may be a cause of slow lifetime degradation of theelectro-optic medium.

Ghosting or similar visual artifacts may be measured optically by aphotometer. In handheld device display screens, two neighboring pixelswith the same target brightness should differ in actual brightness byless than 2 L* (where L* has the usual ICE definition:L*=116(R/R ₀)^(1/3)−16where R is the reflectance and R₀ is a standard reflectance value),preferably less than 1 L*, and ideally less than 0.3 L* to avoid userobjection.

If a remnant voltage decays slowly and is nearly constant, then itseffect in shifting the waveform does not vary from image update toupdate and may actually create less ghosting than a remnant voltage thatdecays quickly. Thus the ghosting experienced by updating one pixelafter 10 minutes and another pixel after 11 minutes is much less thanthe ghosting experienced by updating one pixel immediately and anotherpixel after 1 minute. Conversely, a remnant voltage that decays soquickly that it approaches zero before the next update occurs may inpractice cause no detectable ghosting. Accordingly, for practicalpurposes, remnant voltages that are greater than about 0.2 V for aduration of between 10 ms and one hour, and most specifically between 50ms and 10 minutes, give rise to most concern.

As will be evident from the discussion above, the effects of remnantvoltage are reduced by minimizing the remnant impulse. As shown in FIG.1, this can be accomplished by reducing the peak remnant voltage or byincreasing the decay rate. In theory, it might be predicted that if itwere possible to measure remnant voltage instantaneously and perfectlyafter the completion of a drive pulse, the peak remnant voltage would benearly equal in magnitude but opposite in sign to the voltage of thedrive pulse. In practice, a good deal of the remnant voltage appears todecay so quickly (e.g. less than 20 ms) that the “peak” remnant voltagemeasured experimentally is much smaller. Thus, the “peak” remnantvoltage may be reduced in practice by either (1) operating the displayat a lower voltage or (2) increasing the very fast decay that occurswithin the initial milliseconds after an image update and which resultsin very low remnant impulse. In essence, other than operating at a lowervoltage, one main way to reduce remnant impulse is to increase decayrates.

There are multiple potential sources of remnant voltage. It is believed(although this invention is in no way limited by this belief), that aprimary cause of remnant voltage is ionic polarization within thematerials of the various layers forming the display.

Such polarization occur in various ways. In a first (for convenience,denoted “Type I”) polarization, an ionic double layer is created acrossor adjacent a material interface. For example, a positive potential atan indium-tin-oxide (“ITO”) electrode may produce a correspondingpolarized layer of negative ions in an adjacent laminating adhesive. Thedecay rate of such a polarization layer is associated with therecombination of separated ions in the lamination adhesive layer. Thegeometry of such a polarization layer is determined by the shape of theinterface, but is typically planar in nature.

In a second (“Type II”) type of polarization, nodules, crystals or otherkinds of material heterogeneity within a single material can result inregions in which ions can move or less quickly than the surroundingmaterial. The differing rate of ionic migration can result in differingdegrees of charge polarization within the bulk of the medium, andpolarization may thus occur within a single display component. Such apolarization may be substantially localized in nature or dispersedthroughout the layer.

In a third (“Type III”) type of polarization, polarization may occur atany interface that represents a barrier to charge transport of anyparticular type of ion. An important example of such an interface in amicrocavity electrophoretic display is the boundary between theelectrophoretic suspension including the suspending medium and particles(the “internal phase”) and the surrounding medium including walls,adhesives and binders (the “external phase”). In many electrophoreticdisplays, the internal phase is a hydrophobic liquid whereas theexternal phase is a polymer, such as gelatin. Ions that are present inthe internal phase are typically insoluble and non-diffusible in theexternal phase and vice versa. On the application of an electric fieldperpendicular to such an interface, polarization layers of opposite signwill accumulate on either side of the interface. When the appliedelectric field is removed, the resulting non-equilibrium chargedistribution will result in a measurable remnant voltage potential thatdecays with a relaxation time determined by the mobility of the ions inthe two phases on either side of the interface.

Polarization typically occurs during a drive pulse. Typically, eachimage update is an event that affects remnant voltage. A positivewaveform voltage can create a remnant voltage across an electro-opticmedium that is of the same or opposite polarity (or nearly zero)depending on the specific electro-optic display, as discussed below.

It will be evident from the foregoing discussion that polarizationoccurs at multiple locations within the electrophoretic or otherelectro-optic display, each location having its own characteristicspectrum of decay times, principally at interfaces and at materialheterogeneities. Depending on the placement of the sources of thesevoltages (in other words, the polarized charge distribution) relative tothe electro-active component (for example, the electrophoreticsuspension), and the degree of electrical coupling between each kind ofcharge distribution and the motion of the particles through thesuspension, or other electro-optic activity, various kinds ofpolarization will produce more or less deleterious effects. Since anelectrophoretic display operates by motion of charged particles, whichinherently causes a polarization of the electro-optic layer, in a sensea preferred electrophoretic display is not one in which zero remnantvoltages are always present in the display, but rather one in which theremnant voltages do not cause objectionable electro-optic behavior.Ideally, the remnant impulse will be minimized and the remnant voltagewill decrease below 1 V, and preferably below 0.2 V, within 1 second,and preferably within 50 ms, so that that by introducing a minimal pausebetween image updates, the electrophoretic display may effect alltransitions between optical states without concern for remnant voltageeffects. For electrophoretic displays operating at video rates or atvoltages below +/−15 V these ideal values should be correspondinglyreduced. Similar considerations apply to other types of electro-opticdisplay.

To summarize, remnant voltage as a phenomenon is at least substantiallya result of ionic polarization occurring within the display materialcomponents, either at interfaces or within the materials themselves.Such polarizations are especially problematic when they persist on ameso time scale of roughly 50 ms to about an hour. Remnant voltage canpresent itself as image ghosting or visual artifacts in a variety ofways, with a degree of severity that can vary with the elapsed timesbetween image updates. Remnant voltage can also create a DC imbalanceand reduce ultimate display lifetime. The effects of remnant voltage aretherefore usually deleterious to the quality of the electrophoretic orother electro-optic device and it is desirable to minimize both theremnant voltage itself, and the sensitivity of the optical states of thedevice to the influence of the remnant voltage.

Several approaches to reducing or eliminating ghosting and visualartifacts resulting from remnant voltage are described in previous E Inkpatent applications. For example, the aforementioned 2003/0137521 andcopending application Ser. No. 10/879,335 describe so-called “railstabilized” drive schemes in which the electro-optic medium isperiodically driven to one of the “optical rails” (the two extremeoptical states of the electro-optic medium) where a small remnantvoltage does not have an appreciable effect on the optical state.Copending application Ser. No. 10/837,062, filed Apr. 30, 2004(Publication No. 2005/0012980) describes controlling the capsule heightand pigment level of an electrophoretic medium so that when switching toblack and white a small remnant voltage will not cause a noticeableoptical change.

While such approaches are useful for monochrome displays, they do notaddress the root cause of remnant voltage. In addition, while somewhathelpful for gray scale or color displays, these approaches do notcompletely solve the problem of addressing the system to gray levels,because gray levels in electrophoretic displays are typically dependenton mixing fractions of white and black particles without benefit of aphysical wall to correct for differences in particle speed, andtherefore gray scale addressing is typically more susceptible to smalldifferences between the target waveform and the actual voltageexperienced by the electrophoretic medium.

In the method described in the aforementioned 2003/0137521, a remnantvoltage is measured, and a corrective balancing impulse is appliedeither immediately after each image transition, or periodically, toachieve a zero remnant voltage state. This is helpful for bothmonochrome and grayscale addressing. However, it is not always practicalto measure remnant voltage using the means described in theaforementioned 2003/0137521.

The present invention seeks to provide additional addressingmethodologies for electro-optic displays which will reduce ghostingcaused by remnant voltage but which will not require measurement ofremnant voltage at the pixel level. The present invention also seeks toprovide additional addressing methodologies for electro-optic displaysthat do measure remnant voltage, but which are improved over theaforementioned method, as well as alternative means of measuring remnantvoltage. The methods of the present invention may be useful inelectro-optic displays other than electrophoretic displays. The presentinvention also seeks to provide electro-optic materials, manufacturingmethods, and designs that will minimize remnant voltage. Reducingremnant voltage may be accomplished by reducing peak remnant voltage,accelerating the rate of voltage decay, or any combination thereof.

SUMMARY OF THE INVENTION

This invention provides improved addressing methods for electrophoreticand other electro-optic displays that exhibit remnant voltage. Thisinvention also provides improved display electronics for electrophoreticand other electro-optic displays that exhibit remnant voltage.

In one aspect, this invention provides a method of driving a bistableelectro-optic display having a plurality of pixels each of which iscapable of displaying at least two gray levels. The method comprisesapplying to each pixel of the display a waveform determined by theinitial and the final gray level of the pixel. For at least onetransition from a specific initial gray level to a specific final graylevel, at least first and second waveforms differing from each other areavailable. According to this aspect of the invention, the remnantvoltage of a pixel undergoing the transition is determined prior to thetransition, and the first or second waveform is used for the transitiondepending upon the determined remnant voltage.

This aspect of the present invention may hereinafter for convenience bereferred to as the “waveform selection” method of the invention. In apreferred variant of this waveform selection method (hereinafter forconvenience called the “dwell time waveform selection” method), theselection of the first or second waveform for use in the specifiedtransition is based upon the dwell time of the relevant pixel, i.e., theperiod for which the relevant pixel has been in its initial gray levelprior to the transition. The first waveform is used if the pixel hasbeen in its initial gray level for less than a predetermined interval,and the second waveform is used if the pixel has been in its initialgray level of more than the predetermined interval. The dwell timewaveform selection method may of course make use of more than twowaveforms for the same transition. Thus, in one form of the dwell timewaveform selection method, for the relevant transition, at least first,second and third waveforms, all different from one another, are used,the first waveform being used if the pixel has been in its initial graylevel for less than a first predetermined interval, the second waveformbeing used if the pixel has been in its initial gray level for more thanthe first predetermined interval but less than a second predeterminedinterval, and the third waveform being used if the pixel has been in itsinitial gray level for more than the second predetermined interval. Thefirst and second predetermined intervals will of course vary with thespecific waveforms and electro-optic display being used; however, thefirst predetermined interval may be in the range of about 0.3 to about 3seconds, and second predetermined interval in the range of about 1.5 toabout 15 seconds.

The waveform selection method of the present invention may be carriedout using a look-up table, as described in the aforementioned2003/0137521. Thus, the waveform selection method may comprise:

-   -   storing a look-up table containing data representing, for each        possible transition between gray levels of a pixel, the one or        more waveforms to be used for that transition;    -   storing initial state data representing at least an initial        state of each pixel;    -   storing dwell time data representing the period for which each        pixel has remained in its initial state;    -   receiving an input signal representing a desired final state of        at least one pixel of the display; and    -   generating an output signal representing the waveform necessary        to convert the initial state of said one pixel to the desired        final state thereof, as determined from the look-up table, the        output signal being dependent upon the initial state data, the        dwell time date and the input signal.

Such a “look-up table waveform selection method of the invention maymake use of any of the optional aspects of the look-up table method, asdescribed in the aforementioned 2003/0137521, Ser. Nos. 10/814,205 and10/879,335, and, without prejudice to the generality of the foregoingstatement, may specifically make use of the prior state, temperaturecompensation and lifetime compensation aspects of the look-up tablemethod as described in these copending applications. Thus, the look-uptable waveform selection method may comprise storing data representingat least one prior state of each pixel prior to the initial statethereof, and generating the output signal dependent upon both the atleast one prior state and the initial state of the relevant pixel. Thelook-up table waveform selection method may also comprise receiving atemperature signal representing the temperature of at least one pixel ofthe display and generating the output signal dependent upon thetemperature signal. The look-up table waveform selection method may alsocomprise generating a lifetime signal representing the operating time ofthe relevant pixel and generating the output signal dependent upon thelifetime signal.

This invention also provides a device controller intended for use incarrying out the look-up table waveform selection method of theinvention, and thus for controlling a bistable electro-optic displayhaving a plurality of pixels, each of which is capable of displaying atleast two gray levels. The controller comprises:

-   -   storage means arranged to store look-up table data representing,        for each possible transition between gray levels of a pixel, one        or more waveforms to be used for that transition, at least one        transition having at least two different waveforms associated        therewith, the storage means also being arranged to store        initial state data representing at least an initial state of        each pixel and dwell time data representing the period for which        each pixel has remained in its initial state;    -   input means for receiving an input signal representing a desired        final state of at least one pixel of the display;    -   calculation means for determining, from the input signal, the        initial state data, the dwell time data and the look-up table,        the waveform required to change the initial state of said one        pixel to the desired final state; and    -   output means for generating an output signal representative of        said waveform.

In such a “look-up table waveform selection controller”, the storagemeans may also be arranged to store prior state data representing atleast one prior state of each pixel prior to the initial state thereof,and the calculation means may be arranged to determine the waveformdependent upon the input signal, the initial state date, the dwell timedata, the prior state data and the look-up table. The input means may bearranged to receive a temperature signal representing the temperature ofat least one pixel of the display, and the calculation means may bearranged to determine the waveform dependent upon the input signal, theinitial state data, the dwell time data and the temperature signal. Thecontroller may further comprise lifetime signal generation meansarranged to generate a lifetime signal representing the operating timeof the relevant pixel, the calculation means determining the waveformfrom the input signal, the initial state data, the dwell time data andthe lifetime signal.

In another aspect, this invention provides an electro-optic displaycomprising a layer of electro-optic material and voltage supply meansfor applying a voltage not greater than a predetermined value in eitherdirection across the layer of electro-optic material, wherein theelectro-optic material has a threshold voltage which is greater thanzero but less than about one third of the predetermined value.

This aspect of the invention may hereinafter for convenience be calledthe “low threshold” display of the invention. Such a low thresholddisplay is intended to reduce the effect of remnant voltages on thedisplay. In such a low threshold display, the electro-optic material mayhave a threshold voltage which is not less than about one-fiftieth butless than about one third of the predetermined value. The electro-opticmaterial in such a low threshold display may of any of the typespreviously described; however, the low threshold display is especiallyintended to use a particle-based electrophoretic material comprising asuspending fluid and a plurality of electrically charged particles heldin the suspending fluid and capable of moving therethrough onapplication of a voltage across the layer of electro-optic material. Theelectrophoretic material may be, for example, an encapsulatedelectrophoretic material, a polymer-dispersed electrophoretic materialor a microcell electrophoretic material. The suspending fluid may beliquid or gaseous.

In another aspect, this invention provides an electrophoretic mediumcomprising a suspending fluid, a plurality of a first type ofelectrically-charged particle held in the suspending fluid and capableof moving therethrough on application of an electric field to theelectrophoretic medium, and a plurality of a second type ofelectrically-charged particle held in the suspending fluid and capableof moving therethrough on application of an electric field to theelectrophoretic medium, the particles of the second type having a chargeof an opposite polarity to the particles of the first type, wherein thetotal charge on the particles of the second type is from about one-halfto about twice the total charge on the particles of the first type.

This aspect of the invention may hereinafter for convenience be calledthe “charge balanced dual particle electrophoretic medium” of theinvention. The electrophoretic medium may be, for example, anencapsulated electrophoretic medium, a polymer-dispersed electrophoreticmedium or a microcell electrophoretic medium. The suspending fluid maybe liquid or gaseous. Desirably, such a charge balanced dual particleelectrophoretic medium displays a remnant voltage of less than about 1Volt, and preferably less than about 0.2 Volt, one second after theapplication thereto of a square wave addressing pulse of 15 Volts for300 milliseconds.

In another aspect, this invention provides an electrophoretic mediumcomprising a suspending fluid, a plurality of a first type ofelectrically-charged particle held in the suspending fluid and capableof moving therethrough on application of an electric field to theelectrophoretic medium, and a plurality of a second type ofelectrically-charged particle held in the suspending fluid and capableof moving therethrough on application of an electric field to theelectrophoretic medium, the particles of the second type having a chargeof an opposite polarity to the particles of the first type, the mediumdisplaying a remnant voltage of less than about 1 Volt one second afterthe application thereto of a square wave addressing pulse of 15 Voltsfor 300 milliseconds.

This aspect of the invention may hereinafter for convenience be calledthe “low remnant voltage electrophoretic medium” of the invention.Desirably, such a medium displays a remnant voltage of less than about0.2 Volt one second after the application thereto of a square waveaddressing pulse of 15 Volts for 300 milliseconds.

In another aspect, this invention provides an electrophoretic mediumcomprising a plurality of discrete droplets of a suspending fluiddispersed in a continuous phase, the droplets further comprising aplurality of electrically-charged particles held in the suspending fluidand capable of moving therethrough on application of an electric fieldto the electrophoretic medium, wherein the continuous phase has a volumeresistivity not greater than about one-half of the volume resistivity ofthe droplets, and that both the continuous phase and the droplets havinga volume resistivity of less than about 10¹¹ ohm cm. This aspect of theinvention may hereinafter for convenience be called the “volumeresistivity balanced electrophoretic medium” of the invention and may beof any of types previously discussed; thus, for example, the volumeresistivity balanced electrophoretic medium may be an encapsulatedelectrophoretic medium with capsules walls surrounding the droplets anda polymeric binder surrounding the capsules, a polymer-dispersedelectrophoretic medium or a microcell electrophoretic medium.

The present invention also provides additional improvements and designtechniques for reducing the effects of remnant voltages on electro-opticmedia and displays, especially electrophoretic displays and media. Forexample, this invention provides an electrophoretic display comprising amaterial that has been selected or designed for its ability to carry alow peak remnant voltage or to permit remnant voltages to decay quickly(in short reduced remnant impulse). This invention also provides anelectrophoretic display comprising a material that has been doped,treated, purified, or otherwise processed to reduce its capacity forcarrying remnant voltages. This invention also provides anelectrophoretic display comprising a binder and a laminating adhesivewhich are of similar composition, conductivity or ionic mobility. Thisinvention also provides an electrophoretic display in which theinterface between at least two adjacent components has been treated toreduce remnant voltage, or in which an intervening layer has beenintroduced at least partially to reduce remnant voltage. These aspectsof the invention may hereinafter be collectively termed the “materialselection” invention.

This invention also provides an electrophoretic display comprisingconductive paths within a display pixel for the purpose of reducingremnant voltage. This aspect of the invention may hereinafter be termedthe “conductive paths” invention.

This invention also provides an electrophoretic suspension comprisingtwo species of charged particles, in which the total charge of eachparticle species is selected to reduce remnant voltage. This aspect ofthe invention may hereinafter be termed the “zeta potential” invention.

This invention also provides an electrophoretic suspension comprising anadditive in the suspending fluid that reduces remnant voltage. Thisaspect of the invention may hereinafter be termed the “suspending fluidadditive” invention.

This invention also provides external phase materials for a microcavityelectrophoretic display with reduced remnant voltage. This aspect of theinvention may hereinafter be termed the “microcavity external materials”invention.

This invention also provides methods of manufacturing electrophoreticand other electro-optic displays with reduced remnant voltage, anddisplays comprising means for determining remnant voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

As already mentioned, FIG. 1 of the accompanying drawings is a graphshowing a typical curve of the decay of remnant voltage with time in anelectro-optic display.

FIG. 2 is a schematic side elevation showing the contact circle betweencapsules and surrounding liquid during the coating of an encapsulatedelectrophoretic medium.

FIG. 3A is a schematic side elevation illustrating the forces acting onsparsely coated capsules during the coating of an encapsulatedelectrophoretic medium.

FIG. 3B is a schematic side elevation, similar to that of FIG. 3A butshowing the form of the capsules in the final dried capsule layer as aresult of the forces illustrated in FIG. 3A.

FIGS. 4A and 4B are schematic side elevations, similar to those of FIGS.3A and 3B respectively, showing the forces acting on closely packedcoated capsules and the form of the capsules in the final dried capsulelayers.

DETAILED DESCRIPTION

As already mentioned, the present invention provides several differentimprovements in electro-optic displays and media, and in waveforms andcontrollers for driving such displays. The various aspects of thepresent invention will be described separately (or in related groups)below, although it should be understood that a single display or mediummay make use of more than one aspect of the present invention. Forexample, a single display may contain a volume resistivity balancedelectrophoretic medium of the present invention and use a waveformselection method of the invention to drive this medium.

Methods for Determining Remnant Voltages, and Addressing Methods andControllers for Electro-optic Displays Which Exhibit Remnant Voltages

As already indicated, in view of the deleterious effects of remnantvoltages on the optical performance of electro-optic displays, when adisplay is subject to such remnant voltages it is typically necessary ordesirable to use an addressing method that minimizes the impact ofremnant voltage.

For a given pixel of an electro-optic display, the state of the remnantvoltage is greatly affected by the “image history”, i.e., the electricfields that have been applied previously, and is thus affected byparameters such as the waveforms used, the electric field intensity, andthe elapsed times between successive image updates.

One helpful class of addressing methods described in the aforementioned2003/0137521 and Ser. No. 10/249,973 employs knowledge of the previousimage data. A look-up table is employed in which, for example, thewaveform for a black pixel being switched to white may be different,depending on whether the black pixel had previously been white, or hadpreviously been gray (the transition from gray presumably being adifferent waveform that would have created a different amount of remnantvoltage). Practically it has been found that such “prior n-state lookuptables” do tend to reduce ghosting attributable to remnant voltage.

There are, however, several disadvantages to this approach. Firstly,while the previous optical states are tracked, in some cases thealgorithm used does not take into account the elapsed time between eachimage transition (change of optical state), and as a result the valueschosen for the look-up table must be selected with some usage model inmind, for example, an update on average once per second. Secondly, thismethod requires additional memory, and to achieve higher accuracy, thesize of the look-up table must be increased and the amount of memoryrequired goes up further, especially as n increases past 2 or 3. Asdiscussed in the aforementioned applications, in some cases the verylarge look-up tables required may be difficult to accommodate inportable devices.

In accordance with the waveform selection method of the presentinvention, an alternative approach is now proposed in which the remnantvoltage of each pixel is first determined (or estimated by use ofvarious parameters known to be related to remnant voltage), andthereafter one of two or more waveforms is selected based at least inpart on the determined or estimated remnant voltage. Such a waveformselection method may make use of several possible approaches toestimating or predicting the remnant voltage based on known or measureddisplay characteristics. A waveform selection method may also involvedirect measurement of remnant voltage.

In an exhaustive method, the complete update history of each pixel maybe recorded including both voltage applied and the elapsed times betweenimage updates. A decay model is used to forecast the remnant voltageremaining from each previous update. Updates that occur a sufficientlylong time (typically about 10 minutes) before the transition beingconsidered may be ignored and their history erased because theircontribution to remnant voltage level has been reduced essentially tozero. The remnant voltage of the pixel may then be modeled as theaggregate of remnant voltages from each previous relevant update.

In practice, a preferred approach requiring less memory is to track asingle remnant voltage value and time stamp for each pixel. Prior toeach image update, the remnant value for each pixel is reduced by anamount determined by the decay function of the display and the timestamp for the pixel is updated. After each update, the remnant voltagevalue is increased or decreased by an amount based on the actualwaveform used, and the time stamp is updated. In this way, remnantvoltage is tracked at all times but only two data values have to bestored per pixel.

The decay functions and change functions may be calculated in anysuitable manner, such as by logical computation based on a formula anddata parameters, through analog logic device, or by a look-up table withadequate gradations for the display application. The actual updating ofthe stored remnant voltage and time stamp values may occur in anysuitable manner, such as a single step combining the results of bothcalculations. If a waveform used for an image update comprises a seriesof pulses spread over a long period (e.g. 300-1000 msec), it may beadvantageous to update remnant voltage and/or time stamp values atintervals during the image update itself.

The decay function for a given display is highly sensitive to manyfactors, such as the materials, manufacturing methods, and system designfeatures used in the display. Hence, it is necessary to vary the decayfunction or function parameter values for different electro-opticdisplays. In practice, due to the complexity of electro-optic media anddisplays, it has been found most useful to measure the display systemexperimentally for remnant voltage response and decay against a seriesof applied voltages and to thereby create a look-up table or to fit afunction to the data. It may be helpful to repeat this measuring stepperiodically in the manufacturing process, for example when switching tonew material set or making a batch change. It may also be helpful toindividually characterize the decay function for each display after itis assembled, and to record the resulting parameters in a displaycontroller.

The remnant voltage decay and response function for a given display maybe affected by environmental factors such as temperature and humiditylevel. A sensor or user-selectable input value may be added to thedisplay (or device containing the display) to track such factors. Thus,it may be advantageous to use a remnant response and decay function orlook-up table that allows for these environmental parameters. It mayalso be advantageous to update the remnant voltage value regularly (e.g.every 30-300 seconds) regardless of whether the display has been updatedso that the stored values allow for environmental changes such astemperature and humidity, and remain accurate.

If the pixels in a display are small, the overall remnant voltage on apixel may be significantly affected by the remnant voltages of itsneighbors. Accordingly, remnant voltage updating functions may be usedthat allow for lateral field effects, or a pre- or post-processingalgorithm may be introduced to allow for such effects. Again, it may beuseful to update the remnant voltage value periodically for each pixelin response to the remnant voltage values of its immediate neighbors,and to thereby achieve an adequately accurate estimate of actual remnantvoltage values.

The foregoing discussion has focused on methods for estimating theremnant voltage based on system inputs and characteristics. Analternative approach is to measure the remnant voltage directly.Techniques for sensing the state of an electrophoretic display aredescribed in U.S. Pat. No. 6,512,354. Similar techniques may be used forsensing the remnant voltage in other electro-optic displays. Theaforementioned 2003/0137521 specifically describes the use ofcomparators to measure remnant voltage. Direct measurement of remnantvoltage may be performed prior to each image update or periodically toupdate and correct the data values described above.

Estimating and direct measuring methods may be used together. Forexample, an estimating method may be used at each image update, but theremnant voltage value may be updated periodically based on actualmeasurement. Because the remnant voltage response and decay rate maychange over the working lifetime of a display, which may be a period ofyears, it can be advantageous for display controller software to tracksuch changes and to use an adaptive algorithm, for example a Bayesianalgorithm, that updates its predictive parameters based on actual data.

Similarly, an estimating method may be used for each pixel, the remnantvoltage value may be directly sensed at one or more test pixels, and theremnant voltage value for the remaining pixels adjusted, at least inpart, based on the difference between the estimated and measure remnantvoltage for the test pixels. The test pixels may or may not be pixelsvisible to an observer of the display.

In at least some cases, the remnant voltage characteristics of a displaymay be highly sensitive to the electrical properties of one or morespecific layers of the display, for example, an adhesive layer.Accordingly, the foregoing approaches to estimating or measuring remnantvoltage may be modified to estimate or measure the electricalcharacteristics of the specific layer (such as the adhesive layer)having a major effect on remnant voltage characteristics, and to modifythe algorithms for the remnant values of the pixels appropriately. Asensor may be used to probe the specific layer of the display and may ormay not probe material associated with visible pixels. Furthermore, aphysical sample of the material of the relevant layer may be providedoutside the display as part of a sensor that incorporates the materialdirectly to measure its responses and changes over time.

Remnant Voltage Aware Waveforms and Addressing Methods

Having estimated or measured the remnant voltage (or a proxy variable)by the above or any other suitable methods, in accordance with thewaveform selection method of the present invention, an addressing methodis selected based at least in part on the estimated or measured currentremnant voltage or proxy variable. The addressing method may be chosenbased on the remnant voltage of a specific pixel, the remnant voltage ofthe pixel and the surrounding pixels, or on the overall remnant voltageacross all or a portion of the display larger than one pixel and itsimmediate neighbors.

Various methods may be used to modify a standard waveform (i.e., awaveform which is not remnant voltage aware) to allow for remnantvoltage of a specific pixel or group of pixels. For example, the remnantvoltage may be subtracted from the desired waveform and a reducedvoltage applied, so that the effective waveform experienced by the pixelis the original desired waveform. Alternatively, a scaling factor orother transformation may be applied to the waveform. Alternatively, thevoltage levels in the waveform may be held unchanged, but theirdurations may be adjusted. For example, if a standard waveform requiresa 10 V 50 ms pulse, but the pixel has a 2 V remnant voltage, the pulsemay instead be 40 ms at 10 V, 50 ms at 8 V, 44.7 ms at 8.94 V, or eventwo pulses of 20 ms at 10 V with an intervening pause of 10 ms at 0 V(for simplicity these examples do not take account of the decay rate ofthe 2 V remnant voltage and could be adjusted more precisely to matchthe expected remnant impulse decline from an initial value of 2 V).These waveforms may also be adjusted to allow for the fact that it isnot necessarily ideal for the net impulse to be exactly constant, sincethe electro-optic medium may have a slight threshold or be otherwiseasymmetric in its optical response to the voltage or duration of a pulsein a waveform.

Direct calculation of waveform adjustments in this manner may impose asignificant overhead on the display controller. To reduce such overhead,the controller may instead select an addressing waveform, algorithm,formula or look-up table from a series of options, each associated witha range of remnant voltage values. Thus, the waveform selection methodof the present invention extends to the selection of from among two ormore essentially equivalent waveforms (waveforms that do notsubstantially differ in the final optical state of the pixel after thewaveform has been completed) so as to minimize the change in aggregateremnant voltage within a pixel (i.e., to yield a very low remnantvoltage waveform). The optimum waveform may be determined by modelingthe decay rates of the electro-optic medium or by direct experimentationand a process of tuning and optimization for the waveforms. The waveformselection method of the present invention also extends to selectingamong essentially equivalent waveforms that generate equivalent ornon-minimal remnant voltages, choosing the waveform that brings the netremnant voltage of a given pixel closer to zero; such a waveform may becalled an “off-setting remnant voltage waveform”.

As described in the aforementioned Ser. Nos. 10/814,205 and 10/879,335,it is possible, and often desirable, to use drive schemes in which thewaveforms used for individual transitions are DC-balanced (as opposed tothe overall drive scheme being DC-balanced). In other cases, specificparts of a waveform may be DC-balanced even if the entire waveform isnot DC-balanced; examples are shake-up pulses, blanking pulses (seebelow), and many rail-stabilized addressing methods. During suchDC-balanced waveform sequences, as part of a sequence which involves apixel being driven to both its extreme optical states (hereinafterassumed for convenience to be black and white) the controller may selectin which direction to switch first, towards white or towards black. Whena switch to one extreme optical state is followed by a switch to theother extreme optical state, the second switch will normally have agreater impact on remnant voltage simply because it occurs later in timeand the effects of remnant voltage decay with time. Thus, in aDC-balanced waveform sequence, selecting whether to switch towards blackor white first can determine whether the remnant voltage for a givenpixel is slightly increased or decreased. This is another example of anoffsetting remnant voltage waveform of the invention.

Some drive schemes require a periodic (typically every 10 minutes or so,or every image update) blanking pulse which drives a pixel to bothextreme optical states; see, for example, the aforementioned2003/0137521. For example, the blanking pulse can switch the display toall-white then all-black, or to all-black then to all-white. Inaccordance with the waveform selection method of the present invention,a choice between these alternatives can be made to reduce remnantvoltage and thus to reduce perceived ghosting. Alternatively, bydetermining whether the pixels of the display have, overall, positive ornegative remnant voltages, the total remnant voltage on the display maybe reduced by choosing the appropriate blanking sequence (black/white orwhite/black) without increasing the image update time. In a variation,the decision as to which optical rail (extreme optical state) to hitfirst is not based on the aggregate remnant voltage but is based on thenumber of pixels that have high remnant voltage in either direction.More generally, any suitable algorithm may be used to determine to whichrail the medium will be driven first in order to minimize outliers orother distracting visual artifacts of the display caused by remnantvoltages, given the user preferences for the targeted application.

If desired, the algorithm may also provide for introducing an additionalblanking sequence (white-black-white or white-black-white-black) whenthe remnant voltages are extreme in one or both directions. It will beapparent that the voltage level of the blanking pulse on each pixelcould be modified instead of its duration.

The waveform selection method of this invention also extends toextending the period of a voltage pulse during the time when theelectro-optic medium is already in an extreme optical state (i.e., is atan optical rail), thereby increasing or decreasing remnant voltagewithout a distracting optical change. An opportunity for such voltagepulse extension exists every time a pixel is brought to an extremeoptical state. The blanking pulses mentioned above are one example. Thewaveform selection method therefore provides for the blanking pulseduration (or voltage) to be varied on a pixel-by-pixel basis. Bylengthening the pulse in either direction on a calculated basis for eachpixel, a net remnant voltage component may be applied, and the totalremnant voltage for that pixel thereby reduced or eliminated. Thus, ablanking pulse could be used to reduce remnant voltage across all pixelsof the display without apparent optical impact. As a practical matter,the degree to which the pulse is lengthened could be quantized, i.e.,the pixels could be grouped into categories based on remnant voltageranges and the same adjustment applied to all the pixels in eachcategory.

So-called “rail stabilized” drive schemes are known (see, for example,the aforementioned 2003/0137521, Ser. Nos. 10/814,205 and 10/879,335),which allow any given pixel to undergo only a limited number oftransitions without touching an optical rail, and thus provide for eachpixel to be switched to one of its extreme optical states on a frequentbasis. For example, to transition a pixel from one gray level to anothergray level, the pixel may be switched first to either a dark or a whitestate (possibly for an extended period) and then a subsequent pulseapplied to reach a desired gray level. Such a transition tends to createpositive or negative remnant voltage by virtue of the long period inwhich the pixel is addressed toward the extreme optical state. Accordingto the waveform selection method of the present invention, the remnantvoltage of the pixel may be minimized by causing the transition to usethe optical rail for which the remnant voltage created by the directionof the switch will tend be opposite in sign to the remnant voltagecarried by the pixel just prior to the transition.

One reason for using a rail-stabilized drive scheme is to mitigate theoptical effects of remnant voltage. The estimate of measurement ofremnant voltage on a pixel-by-pixel level, as described above, canreduce the need for the use of such rail-stabilized drive schemes. Ahybrid approach is to use rail-stabilized methods for pixels in whichremnant voltage is fairly high, but to switch directly to the desiredstate (a direct impulse method) when remnant voltage is low and wouldnot affect the image.

Another approach to reducing remnant voltage is to identify pixels forwhich the remnant voltage is extreme (i.e., has a magnitude greater thansome predetermined value), and, prior to a general image update, toapply an off-setting voltage to such pixels to reduce their remnantvoltages, or otherwise pre-condition the remnant voltage levels acrossthe display. Such pre-conditioning may enable a reduced period of railstabilization and achieve a faster perceived image update time. If theoff-setting voltage is small and applied over a sustained time, or if ittends to lengthen a period of rail stabilization rather than pull theparticles back from the rail, the reduction of remnant voltages may beaccomplished without distracting visual impact.

The above discussion has focused on tracking net remnant voltages andselecting appropriate algorithms for reducing remnant voltages. Anotherparameter of image history of a pixel or display is net DC imbalance. Itwill be apparent to one skilled in the imaging art that most of themethods described above can be modified to track and correct for net DCimbalance, either in combination with, or independently from, anyadjustments for remnant voltage. For example, DC imbalance can be usedwhen determining which optical rail to select first and whatpre-conditioning is appropriate in the above approaches. Also, forexample, even when remnant voltage may be reduced by modifying awaveform, the drive scheme could omit this correction if the pixel isalready DC-imbalanced and would become more imbalanced after theadjustment for remnant voltage. Similarly, any conditioning of thedisplay or adjustment of waveforms to achieve a net DC balance would beallowed for when estimating the remnant voltage across each pixel.

Thus, the waveform selection method of this invention may be generalizedas an addressing method for an electro-optic display capable ofexhibiting a remnant voltage, wherein a data value corresponding toremnant voltage is determined and an addressing waveform is selected atleast in part based on the remnant voltage value. In such a method, timeand remnant voltage values, or data representing each, are typicallyexplicitly tracked. However, it should be recognized that addressingwaveforms for electrophoretic and other electro-optic displays mayaccount for time and remnant voltage values implicitly or approximately.For example, the so-called “prior n-state” addressing methodologiesdescribed the aforementioned 2003/0137521, Ser. Nos. 10/814,205 and10/879,335 and above may not track time, but they do track a history ofprior pixel optical states, and this can be a proxy for time if thedrive scheme designer has some knowledge of typical usage models andcommon elapsed times between image updates. Hence, it is now recognizedthat such methods tend to reduce remnant voltage and thus show improvedghosting behavior.

One practical reason why such methods have previously been used is thatthe display controller in many electro-optic displays does not haveaccess to clock information to track elapsed time between image updates,perhaps because such elapsed time data is most useful for bistabledisplays and few bistable displays have hitherto been commercialized. Ina preferred form of the waveform selection method of the presentinvention, the controller does comprise a clock or equivalent timingmechanism. Alternatively, the controller may be in logical communicationwith an external information source (such as the device which uses thedisplay as its output device) that generates an elapsed time value andprovides this information to the controller. For example, the device mayprovide time information along with a function call to the displaycontroller or along with each new set of image data. Such timeinformation may be quantized (e.g. immediate, 0.5 seconds, 1 second, 2seconds, 10 seconds, 30 seconds, 60 seconds, more than 60 seconds)thereby reducing data bandwidth and yet still providing usefulinformation, especially if the quantized time bands are chosen tocorrespond to the substantially exponential decay of the remnantvoltage.

In general, it is most useful for the controller to receive elapsed timedata for each pixel, since some pixels may not change during an update.However, it is still useful for the controller to receive datacorresponding to the elapsed time since the most recent image update,most recent blanking pulse, or most recent update for a set of pixels.Additionally, the controller may receive data indicating the likelyupdate frequency of the display, for example, a flag indicating whetherthe user is currently entering text, which may require many updates inrapid succession to the whole display or to a defined region thereof.

Another form of approximate correction of remnant voltage is used in thedwell time waveform selection method of the present invention, whichprovides for choice among multiple waveforms to effect an imagetransition, where the selection among the multiple waveforms is based atleast in part on the dwell time of the relevant pixel in its initialgray state, or some proxy for this dwell time. Such time-sensitiveselection among multiple waveforms implicitly accounts for the decay ofremnant voltage with time, even though remnant voltage is not explicitlytracked, estimated or measured.

For example, a specific dwell time waveform selection method of thepresent invention might be applied to a controller for a display withfour gray levels using a drive scheme based on a logical transitiontable with 16 entries, each entry corresponding to the transition fromone gray level (0,1,2,3) to another (0,1,2,3). Selection of the entry isbased on knowledge of the initial and final gray levels of the desiredtransition. Within each entry, there are three possible waveforms. Thecontroller selects the first waveform when the image transition occurswithin 1 second after the prior image update, the second waveform whenthe image transition occurs between 1 and 5 seconds after the priorimage update, and the third waveform when the image transition occursmore than 5 seconds after the prior image update.

In the dwell time waveform selection method, the waveforms may berepresented by look-up tables (as described above), may be modified (orsplit into sub-tables) to allow for variation in environmentalconditions, and may be set in whole or in part during manufacture of thedisplay to include specific parameters of an individual display. Inshort, the waveforms used in this method may include any of the optionalcomponents and variations described in the aforementioned 2003/0137521,Ser. Nos. 10/814,205 and 10/879,335.

From the foregoing it will be seen that, although in the dwell timewaveform selection method of the present invention remnant voltage isnot explicitly tracked, and although the elapsed time may be based onelapsed time since the display was updated and not on elapsed time sincea specific pixel was updated, the dwell time waveform selection methoddoes implicitly approximate both remnant voltage and elapsed pixelupdate time and therefore exhibits improved ghosting behavior over priorart drive schemes.

Materials Selection

As already indicated, the selection of materials for use inelectro-optic displays can have a major influence on the remnantvoltages which exist in such displays during their operation, and henceupon the electro-optic performance of such displays.

Also as discussed above, when used in an electro-optic display certainmaterials exhibit Type I polarization which contributes to remnantvoltage. It is believed (although the invention is in no way limited bythis belief) that this polarization is frequently due to the mobilityand concentration of ions moving through at least one of the componentmaterials.

The speed of decay of remnant voltage may be measured in any specificmaterial by preparing a test cell in which the material is in contactwith the same interfaces as in the proposed display. For example, testcells have been prepared consisting of a controlled thickness oflaminating adhesive coated onto an ITO substrate, and an electric fieldapplied across the laminating adhesive/ITO interface. Remnant voltagepeak values and decay were then measured by opening a charging circuit,and monitoring the voltage across the pixel with a high impedancevoltmeter.

It has been found that laminating adhesives with higher ionic mobilityshow faster remnant voltage decay rate. A preferred lamination adhesivehas a volume resistivity of less than about 10¹¹ ohm cm.

Previous E Ink patent applications, for example the aforementioned U.S.Pat. No. 6,657,772 and Patent Publication No. 2003/0025855, andapplication Ser. No. 10/708,121, filed Feb. 10, 2004 (Publication No.2004/0252360), describe lamination adhesives with controlledresistivities, or which are heterogeneously or anisotropicallyconductive, for example Z-axis adhesives. Such adhesives may provide afurther benefit of reducing remnant voltage.

A lamination adhesive may also exhibit Type II polarization. In testcells, increased adhesive thickness has been found to be associated withhigher remnant voltage. Since polarization at the interfaces should beindependent of the film thickness, this result suggests the existence ofinternal charge polarization sites, characteristic of Type IIpolarization effects. Consequently, care must be taken in the selectionof the adhesive thickness and, in the case of encapsulatedelectrophoretic displays, its morphology around the capsules. The sametest lamination adhesive was heated to drive out suspected impuritiesand crystalline regions. Thereafter it exhibited reduced remnantvoltage.

Type I polarization may occur anywhere in the display where a materialinterface exists. It has been found that, by using the same material forlamination adhesive and binder (i.e., the material used to surround thecapsules and form them into a cohesive layer, as described in many ofthe aforementioned E Ink and MIT patents and applications), an interfaceis eliminated and the remnant voltage is reduced. Therefore, the presentinvention provides an electrophoretic display comprising a microcavitybinder and a laminating adhesive in which the materials are eitheridentical or similar in composition or electrically equivalent inconductivity or ionic mobility. In some cases where the materials aredifferent in composition, it may be desirable to dope the lessconductive material to achieve substantially equal ionic mobility onboth sides of the interface.

Type I polarization at some interfaces can be affected by surfaceroughness. It may be advantageous either to planarize or to introduce atexture to some interfaces, thereby providing a degree ofinterpenetration of the materials on either side of the interface. Thesetechniques may result in either increased polarization at the specificinterface or in decreased polarization, either of which could bebeneficial depending upon the specific display being considered. Forexample, increased polarization at one location that offsetspolarization elsewhere in the display may cause a reduced remnantvoltage across an electro-optic medium. Typically if the interfaceresults in a remnant voltage that is strongly coupled to theelectro-optic medium, then reducing the degree of polarization at theinterface and its decay rate is desirable.

Type I polarization at some surfaces may also be affected by surfacecleanliness. Cleaning of substrates prior to coating and lamination isdesirable in order to achieve consistent electrical behavior.

Conductive Paths in Electrophoretic Layer

In microcavity electrophoretic displays, a cell wall (a term which isused herein to include the capsule wall of an encapsulated display)exists that is electrically in parallel with the electrophoreticinternal phase (the suspending fluid and the electrically chargedparticles). Current, in the form of electrically charged ions, can flowthrough the internal phase or through the cell wall. The cell wall canbe a polymer, such as a gelatin, or any other suitable material. Thecell wall is typically further surrounded by a binder, as mentionedabove. Therefore, some current may flow between the electrodes of thedisplay via the binder or cell wall without flowing through theelectrophoretic internal phase, and thus without contributing to changesin the electro-optic state of the display or a pixel thereof.

In the preferred electrophoretic displays described in theaforementioned E Ink and MIT patents and applications, theconductivities of the cell wall and binder are typically slightly higherthan those of the internal phase. Relaxation of remnant voltage may thusoccur in part through the binder and cell wall.

During the application of an electric field to the electrophoreticmedium, charged particles move toward the two electrodes of the display.If charged particles cluster near the front electrode (the electrodethrough which an observer normally views the display) for a period oftime, corresponding electrons or oppositely charged ions may flowthrough the cell wall and/or binder in response. The charged regionsthus created may create a remnant voltage that affects a subsequentimage update. Consequently, the conductivities and ionic mobilities ofthe cell wall and binder are of importance, as are their morphologies.

The remnant voltage of a specific cell/binder morphology may be measuredby methods similar to those described above for a lamination adhesive.In accordance with the volume resistivity balanced electrophoreticmedium aspect of the present invention, it is preferred that the binderand cell walls have a volume resistivity at least two times less thanthe volume resistivity of the electrophoretic internal phase and thatboth have a volume resistivity of less than about 10¹¹ ohm cm. Moregenerally, in an electrophoretic medium comprising a plurality ofdiscrete droplets of a suspending fluid dispersed in a continuous phase(which may have the form of a single continuous phase in apolymer-dispersed medium, a combination of cell walls and binder in anencapsulated electrophoretic medium, or cell walls only in a microcellelectrophoretic medium), the droplets comprising a plurality ofelectrically-charged particles held in a suspending fluid and capable ofmoving therethrough on application of an electric field to theelectrophoretic medium, it is preferred that the continuous phase have avolume resistivity not greater than about one-half of the volumeresistivity of the droplets, and that both the continuous phase and thedroplets have a volume resistivity of less than about 10¹¹ ohm cm. In apreferred embodiment, the binder and cell walls occupy between about 5and about 20% by volume of the electrophoretic layer (with the remainderbeing the electrophoretic internal phase), and the binder is evenlydistributed among the capsule walls.

Zeta Potential Considerations, and Charge Balanced Dual ParticleElectrophoretic Medium

A preferred type of electrophoretic medium described in many of theaforementioned E Ink and MIT patents and applications is a so-called“opposite charge dual particle” medium, in which the electrophoreticinternal phase contains two different types of particles bearing chargesof opposite polarity (see, for example, the discussion of the differenttypes of electrophoretic media in the aforementioned 2002/0171910). Theamount of charge on each particle may be controlled, for example bysurface modification as described in U.S. Patent Publication No.2002/0185378 (now U.S. Pat. No. 6,870,661), and in copending applicationSer. No. 10/711,829, filed Oct. 7, 2004 (see also the correspondingInternational Application No. PCT/US2004/033188, Publication No. WO2005/036129). The number of particles in each microcavity may also becontrolled in a predictable way by selecting the total amount ofparticles provided in the electrophoretic internal phase prior toencapsulation or filling of microcells. By multiplying the averagecharge per particle by the average number of particles per microcavity,it is possible to estimate the total charge of each type of particle inthe microcavity.

It has been found that if the total charges of the oppositely chargedtypes particle are not approximately balanced, a particularly largepolarization is produced in the polarized microcavity, which induces acorresponding large and slowly decaying polarization in the continuousphase material(s). It has further been found that by varying the nettotal charge of the particle types, it is possible to vary anencapsulated electrophoretic display between a regime in which anelectric field leaves a remnant voltage of the same sign (so that asubsequent update in the opposite direction is retarded), a regime inwhich very little remnant voltage occurs, and a regime in which anelectric field leaves a remnant voltage of the opposite sign (so that asubsequent update in the opposite direction is promoted).

In accordance with the charge balanced dual particle aspect of thepresent invention, it is preferred that neither type of electrophoreticparticle have more than about twice the total charge of the other. It isalso preferred, in accordance with the low remnant voltageelectrophoretic medium aspect of the present invention, that, in anopposite charge dual particle electrophoretic display, the particlecharge, particle mass, and particle mobility be selected so that thedisplay exhibits “Low Remnant Voltage Behavior”, herein defined ashaving a remnant voltage measuring less than about 1 V (and desirablyless than about 0.2 V) exactly 1 second after the application thereto ofa square wave DC addressing pulse of 15 V for 300 milliseconds.

To assess charge balance in an opposite charge dual particleelectrophoretic internal phase, it is helpful to analyze the charge oneach particle relative to its mass (since mass can be easily measured atthe time of manufacture). It is believed, although the invention is inno way restricted by this belief, that the charge to mass ratio may beestimated using the following relationship:q/M is proportional to ζ/d ²  (1)where:

-   -   q=particle charge    -   M=mass    -   ζ=zeta potential (mV)    -   d=particle diameter.

The total net charge of the electrophoretic internal phase shoulddesirably be controlled by careful co-optimization of particle charge,particle mass, particle diameter, and zeta potential.

In a substantially charge-balanced electrophoretic medium exhibiting LowRemnant Voltage Behavior (as defined above), such behavior may typicallycease if any of the following occurs: (a) the average charge on eithertype of particles is changed by about 20% to 100%; (b) the relative massof one type of particle is changed by about 50% to 300%; (c) the averagediameter of one type of particle is changed by about 30% to 200%; and(d) the average mobility of one type of particle is changed by about 20%to 100%.

Suspending Fluid Additives

It has been found that the addition of surfactants to the suspendingfluid of the electrophoretic medium may reduce remnant voltage. Forexample, when single pixel displays were prepared using otherwiseidentical dual particle opposite charge electrophoretic media but inwhich sorbitan trioleate (sold commercially as Span 80) was added to oneof the suspensions, the display containing the sorbitan trioleatedemonstrated reduced remnant voltage.

It is believed, although the invention is in no way restricted by thisbelief, that the surfactant alters the relative charge balance of thetwo types of electrophoretic particles. It is further believed that thesurfactant reduces Type III polarization by modifying charge relaxationrates in the electrophoretic internal phase so that they more closelybalance the corresponding relaxation rates in the external phase.

Thus, this invention provides an electrophoretic display exhibiting LowRemnant Voltage Behavior (as defined above), which behavior ceases ifthe concentration of a surfactant or charge control agent in theelectrophoretic internal phase is changed by about 30% to 200%.

Materials for External Phases of Microcavity Electrophoretic Displays

It is possible to select external phase materials for use in microcavityelectrophoretic displays, or to mix, dope or condition such materials,to achieve desired remnant voltage relaxation rates. As described above,the relaxation rate of an internal phase may be affected by numerousfactors, including the choice of electrophoretic particle(s) and theconcentration of surfactants and charge control agents. One aspect ofthe present invention provides that the external phase materials and theinternal phase materials be balanced (within a factor of 2) inrelaxation rates.

This aspect of the invention provides an electrophoretic displayexhibiting Low Remnant Voltage Behavior (as defined above), whichbehavior ceases if the conductivity of the external phase materials ischanged by about 30% to 200%.

In a typical encapsulated electrophoretic display, a critical externalphase material is the gelatin capsule wall. The conductivity of the wallis significantly affected by moisture. In a preferred embodiment, theelectrophoretic display comprises moisture and is resistant to changesin the relative humidity (RH) of the operating environment. In a furtherpreferred embodiment, the display is conditioned (by placing it in acontrolled humidity environment until it has come to equilibrium and/orby manufacturing the display in a controlled humidity environment) so asto achieve between 20% RH and 55% RH, and preferably 35% RH, for theelectrophoretic layer within the final display.

Thus the invention provides a method of manufacturing an electrophoreticdisplay that comprises RH conditioning the display material. Theelectrophoretic display may also comprise moisture barriers orsubstrates that are impermeable to water.

Low Threshold Electro-optic Displays

A small threshold in an electrophoretic or other electro-optic displaymay be produced in many ways. The threshold can result from attractionsbetween particles and walls, or among particles. The attractions can beelectrical, such as from oppositely charged particles; physical, such asfrom surface tension; or magnetic. A threshold can also result from thenature of a suspending fluid, which may be strongly shear-thinning, orhave an apparent yield stress (such as for a Bingham fluid), orelectro-rheological properties. An additional electrical field, forinstance a field created by in-plane electrodes or a control grid, cansubstitute for a threshold.

For purposes of this application, a threshold is considered present at aparticular voltage level when a square wave DC pulse of 1 secondduration applied to the display at that voltage level results in anoptical change of less than 2 L*.

It is known in the display art that a threshold in an electro-opticmedium can serve as a basis for a passive addressing scheme. Typicallysuch a scheme relies on a threshold equal to half of the switchingvoltage (“V/2”); in some drive schemes, passive addressing can beachieved with a minimum threshold of one-third of the switching voltage(“V/3”).

In contrast, as described above, a threshold of as little as 1 V, ascompared with a switching voltage of ±15 V, can be useful in reducingthe impact of remnant voltages on electro-optic performance.Accordingly, the low threshold display aspect of the present inventionprovides an electro-optic display operating at a voltage not greaterthan ±V, wherein the electro-optic material has a threshold voltagewhich is greater than zero but less than about V/3.

Manufacturing Electrophoretic Displays with Reduced Remnant Voltage

A final aspect of the present invention relates to various improvementsin the manufacture of electrophoretic displays to reduce the remnantvoltages exhibited by the displays thus manufactured.

During the manufacture of encapsulated electrophoretic displays,capsules are typically suspended in a slurry, which comprises thecapsules and a polymeric binder, and may also comprise variousadditives, for example water, plasticizers, pH adjusters, biocides, andsurfactants or charge control agents. For present purposes, such aslurry may be regarded as containing a “binder” consisting of thenonvolatile components of the slurry, excluding the capsule. In somecases, the binder materials may separate during preparation of theslurry, or during shipment and storage, and may not always be mixedadequately prior to coating. As a result, regions of area-to-areaheterogeneity may exist that can cause Type II polarization problems inthe final display. To reduce such problems, it is desirable tothoroughly mix such binder materials through appropriate means such asmixing by propeller blade or on a roll mill for extended periods.

The dried binder material should desirably have uniform electricalcharacteristics such that, following a 15 V voltage pulse applied for300 ms and a 1 second pause, the measured remnant voltage of the bindermaterial itself should be less than about 1 V, and preferably less than0.2 V.

As mentioned above, it is desirable to control the amount of spacebetween capsules that is occupied by binder because this space cancontribute to Type III polarization. Electrodeposition may be used tocontrol capsule spacing directly, as described in copending applicationSer. No. 10/807,594, filed Mar. 24, 2004 (Publication No. 2004/0226820;see also the corresponding International Application PCT/US2004/009421,Publication No. WO 2004/088002). In microcell or photo-patternedelectrophoretic displays, microcavity spacing can be controlleddirectly.

In coated encapsulated electrophoretic displays, dried capsule spacingand morphology are the result of many controllable factors, as discussedin several of the aforementioned E Ink and MIT patents and applications.To summarize, capsule morphology can be adjusted by varying capsule wallthickness and elasticity, the formulation of the coating slurry, thesurface energy of the coating substrate, the height of the coating dieoff the substrate, the amount of coating slurry passing through orpumped through the die onto the substrate, the speed of a substrate web,and the drying conditions of the wet coated film such as temperature,duration and air flow. Useful principles for control of capsule spacingand morphology are described below.

A: Capsule Wall Property Effects on Dried Capsule Shape

Capsule wall properties vary with materials and process variables ofencapsulation, especially mixing speed. The capsule wall shoulddesirably be elastic enough to allow an overall capsule height/diameterratio between 0.33 and 0.5. However the capsule wall should also ideallypermit local variations enabling nearly a 90-degree bend radius on sharpcorners for hexagonal close packing of the capsules on the substrate onto which they are coated, as described for example in U.S. Pat. Nos.6,067,185 and 6,392,785.

It is believed (although the invention is in no way limited by thisbelief) that capsule wall elasticity can be affected by the degree ofcross-linking of the capsule wall material (less cross-linking typicallygiving a more flexible capsule wall) and by the thickness of the wall.Wall thickness is affected by internal phase formulation, gelatin/acacialevels and process parameters. For a given capsule packing pattern,reducing the wall thickness can improve the “aperture ratio” (i.e., thefraction of the area of the electrophoretic medium which undergoeschange of optical state; the areas occupied by the capsule walls cannotundergo such change) of the medium; however, walls that are too thin mayburst easily.

Certain process parameters that have been found important in affectingwall thickness are set out in the Table below. The results shown in theTable were generated through encapsulation experiments done at a 4 Lscale. Also shown in the Table are the relative qualitative rankings forthe wall thickness compared to a standard operating procedure forencapsulation. The standard process conditions for the 4 L encapsulationare acacia level (index at 100% of standard level), pH (4.95),emulsification temperature (40° C.), cooling rate (3 hours), and rate ofinternal phase addition. In the Table, a Rank of 3 denotes walls ofstandard thickness, with 1 denoting a very thin wall and 5 a thick wall.

TABLE Effect on wall thickness Rank Rank Parameter Low High Acacia: 25%variation in mass 3 1.5 pH: 3% variation on pH scale 3 2 EmulsificationTemp: 10% variation 4 3 Cooling Rate: ±2 hours 2 4 Rate of IP addition:Spray/Dropwise 2.5 3.5

pH is a critical parameter for the wall properties, not just in terms ofthe wall thickness but also because the solid content and viscosity ofthe coacervate are quite different at different pH levels. Finally, thetype of gelatin and acacia used may have a dramatic impact on the wallproperties.

B: Binder Evaporation as a Mechanism for Changing Dried Capsule Shape

The effect of binder evaporation varies depending on how closely packedthe capsules were while the coated slurry was still wet. The same binderratio, with the same capsule diameters, may result in either flattened(oblate ellipsoidal) or tall (substantially prismatic) capsulesaccording to wet capsule proximity.

FIG. 2 shows the situation where a capsule/binder slurry has been coatedon a substrate 110 so that the capsules 112 are sparsely coated, i.e.,are separated from one another by gaps comparable to the diameter of thecapsules 112. As shown in FIG. 2, in these circumstances the capsules112 are only partially immersed in the uncured binder 114, so that theportions of the capsules 112 remote from the substrate 110 protrude fromthe layer of binder 114, and the boundary between the binder and eachcapsules is a circle of radius r_(c) around each substantially sphericalcapsule. It can be shown that the downward force exerted on a capsule bysurface tension forces during drying is given by:F=2πσr _(c) sin ψ_(c)where:

-   -   F is the downward force on the capsule;    -   σ is the surface tension of the liquid surrounding the capsule;    -   r_(c) is the diameter or the contact circle which the liquid        makes with the capsule, as illustrated in FIG. 2; and    -   ψ_(c) is the complement to the contact angle of the liquid        surrounding the capsule (i.e., 90°—the contact angle).

From FIG. 2, it will be seen that r_(c) will increase as the capsulesincrease in size and as the level of the surrounding liquid is lowered.

Two extreme cases of the effects of this downward force are shown inFIGS. 3A-3B and 4A-4B respectively of the accompanying drawings. InFIGS. 3A and 4A, arrows A denote evaporation of water from the wetbinder, while arrows B denote the forces on the capsules exerted bysurface tension. In FIGS. 3B and 4B, “C” denotes dried binder. (Notethat throughout FIGS. 3A to 4B, the presence of binder outside the twoillustrated capsules is ignored.) FIGS. 3A and 3B illustrate the effectsof the downward force on sparsely coated wet capsules, i.e., capsulescoated so as to leave gaps between adjacent capsules which are asubstantial fraction of a capsule diameter. From FIGS. 3A and 3B, itwill be seen that the effect of the downward force is to flatten theoriginal spherical capsules into oblate ellipsoids, which typically willtouch each other in the final dried layer, but that little or nodistortion of these ellipsoids occurs by contact between adjacentcapsules. In contrast, FIGS. 4A and 4B illustrate the effects of thedownward force on closely packed coated wet capsules, in which the wetcapsules as coated are in contact with one another. From FIGS. 4A and4B, it will be seen that the effect of the downward force is to forcethe capsules to contact each other over progressively larger areas, sothat in the final dried layer the capsules have substantially the formof polygonal prisms having a height substantially greater than theirwidth; if the wet capsules are hexagonally close packed, as is ideallythe case, the dried capsules will have substantially the form ofhexagonal prisms. It should be noted that if capsules are too sparselypacked, voids may be left between the capsules as they dry and tend toassociate into clusters.

C: Slurry Preparation Effects on Dried Capsule Shape

The pH level of the capsules may affect dried capsule shape. As pH isincreased, the charge on gelatin changes, affecting the attraction ofthe gelatin to the substrate (typically an ITO surface) on which thecapsules are coated and making it harder or easier for capsules to shiftlocation. Choice of substrate surface energy by changing substrates mayaffect this relationship.

Surfactant level affects the adhesion (“stickiness”) of capsules to eachother and possibly to the binder. A more surface active surfactantweakens surface tension and should reduce the surface tension forcesacting on the capsules during drying. A less surface active formulationmay help flatten capsules.

Binder ratio is a critical factor affecting dry capsule shape. Lowerbinder ratios result in rounder capsules. A binder ratio of 2:1 (i.e.,two parts by weight of capsules to one part by weight of binder) issufficient to fully surround each capsule as a perfect sphere when driedand hence results in the least flattened capsules. Lower binder ratiosallow the binder, when dry, to fill the interstices between capsules. Abinder ratio of 8:1 is adequate to achieve flat capsules or heightened(polygonal) capsules depending on coating conditions.

D: Coating Parameter Effects on Dried Capsule Shape

As described above, the coating process should optimally deposit wetcapsules a predetermined distance apart. Critical parameters inachieving such desired spacing include coating speed, die type, dieheight, and slurry flow rate.

Experimentally, it has been found that increasing the slurry flow ratein a coater, while holding all other parameters constant, tends toincrease coating weight, with the result that the wet capsules areplaced closer together. This could result in some capsules being tooclosely spaced, resulting in less flattened/more heightened driedcapsules.

In related coating experiments, decreasing the gap in the die to a lowvalue (e.g., 40-50 μm) brought the die height to a value comparable tothe size of the wet capsules being used. At this die height, a monolayerof capsules was virtually assured, but the packing was usually verytight. Coatings at lower die heights tended to result in heighteneddried capsules. Ideally, the capsules would be coated “almost touchingtogether” but not packed together when wet.

E. Drying Parameter Effects on Dried Capsule Shape

Experimentally, it has been found that a conveyor oven drying at 60° C.for 2 minutes is able to create capsule-containing films with flattened,heightened and spherical dry capsules. Attempting to dry the capsulestoo quickly may cause a “skin” to form on the top of the binder; thisskin traps moisture within the binder and causes the film to dry veryslowly.

The rate of air flow while drying affects the rate of evaporation andaffects whether evaporative gases are trapped among the capsules. It isdifficult to achieve good success in drying without adequateventilation; air flow across the binder is helpful.

The foregoing description has emphasized the application of theinvention to electrophoretic displays. Such electrophoretic displays maybe of any type and still benefit from at least some aspects of thisinvention. Thus, the displays may include microcavity electrophoreticdisplays, such as encapsulated, microcell, microcup, andpolymer-dispersed displays; electrophoretic displays using one or morespecies of particles (except of course for those aspects of theinvention specific to dual particle electrophoretic displays);electrophoretic displays using clear or dyed suspending fluids;electrophoretic displays comprising oil-based and gaseous suspendingmedia; flexible and rigid electrophoretic displays; electrophoreticdisplays addressed by non-linear devices (such as thin filmtransistors), by passive means (such as a control grid) and by directdrive; electrophoretic displays operating by lateral or in-plane motionof the electrophoretic particles, by vertical or electrode-to-electrodemotion or any combination thereof; and full-color, spot-color andmonochrome electrophoretic displays.

Finally, it is again emphasized that although this invention has beenprincipally described as applied to electrophoretic displays, manyaspects thereof are applicable to any electro-optic display or mediumcapable of a remnant voltage, with particular importance for bistableelectro-optic displays.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of the presentinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beconstrued in an illustrative and not in a limitative sense.

What is claimed is:
 1. A method of driving a bistable electro-opticdisplay having a plurality of pixels each of which is capable ofdisplaying at least two gray levels, the method comprising applying toeach pixel of the display a waveform determined by the initial and thefinal gray level of the pixel, wherein, for at least one transition froma specific initial gray level to a specific final gray level, a firstwaveform is used if the pixel has been in its initial gray level forless than a predetermined interval, and a second waveform, differentfrom the first waveform, is used if the pixel has been in its initialgray level for more than the predetermined interval.
 2. A methodaccording to claim 1 wherein, for said at least one transition, first,second and third waveforms, all different from each other, are used, thefirst waveform being used if the pixel has been in its initial graylevel for less than a first predetermined interval, the second waveformbeing used if the pixel has been in its initial gray level for more thanthe first predetermined interval but less than a second predeterminedinterval, and the third waveform being used if the pixel has been in itsinitial gray level for more than the second predetermined interval.
 3. Amethod according to claim 2 wherein the first predetermined interval isin the range of from about 0.3 to about 3 seconds, and the secondpredetermined interval is in the range of from about 1.5 to about 15seconds.
 4. A method according to claim 1 comprising: storing a look-uptable containing data representing, for each possible transition betweengray levels of a pixel, the one or more waveforms to be used for thattransition; storing initial state data representing at least an initialstate of each pixel; storing dwell time data representing the period forwhich each pixel has remained in its initial state; receiving an inputsignal representing a desired final state of at least one pixel of thedisplay; and generating an output signal representing the waveformnecessary to convert the initial state of said one pixel to the desiredfinal state thereof, as determined from the look-up table, the outputsignal being dependent upon the initial state data, the dwell time dateand the input signal.
 5. A method according to claim 4 furthercomprising storing data representing at least one prior state of eachpixel prior to said initial state thereof, and wherein said outputsignal is generated dependent upon both said at least one prior stateand said initial state of said one pixel.
 6. A method according to claim4 further comprising receiving a temperature signal representing thetemperature of at least one pixel of the display and generating saidoutput signal dependent upon said temperature signal.
 7. A methodaccording to claim 4 further comprising generating a lifetime signalrepresenting the operating time of said pixel and generating said outputsignal dependent upon said lifetime signal.
 8. A device controller forcontrolling a bistable electro-optic display having a plurality ofpixels, each of which is capable of displaying at least two gray levels,the controller comprising: storage means arranged to store look-up tabledata representing, for each possible transition between gray levels of apixel, one or more waveforms to be used for that transition, at leastone transition having at least two different waveforms associatedtherewith, the storage means also being arranged to store initial statedata representing at least an initial state of each pixel and dwell timedata representing the period for which each pixel has remained in itsinitial state; input means for receiving an input signal representing adesired final state of at least one pixel of the display; calculationmeans for determining, from the input signal, the initial state data,the dwell time data and the look-up table, the waveform required tochange the initial state of said one pixel to the desired final state;and output means for generating an output signal representative of saidwaveform.
 9. A controller according to claim 8 wherein the storage meansis also arranged to store prior state data representing at least oneprior state of each pixel prior to the initial state thereof, and thecalculation means is arranged to determine said waveform dependent uponthe input signal, the initial state date, the dwell time data, the priorstate data and the look-up table.
 10. A controller according to claim 8wherein the input means is arranged to receive a temperature signalrepresenting the temperature of at least one pixel of the display, andthe calculation means is arranged to determine said waveform dependentupon the input signal, the initial state data, the dwell time data andthe temperature signal.
 11. A controller according to claim 8 furthercomprising lifetime signal generation means arranged to generate alifetime signal representing the operating time of said pixel, thecalculation means determining said waveform from the input signal, theinitial state data, the dwell time data and the lifetime signal.
 12. Amethod of driving a bistable electro-optic display having a plurality ofpixels each of which is capable of displaying at least two gray levels,the method comprising applying to each pixel of the display a waveformdetermined by the initial and the final gray level of the pixel,wherein, for at least one transition from a specific initial gray levelto a specific final gray level, at least first and second waveformsdiffering from each other are available, and the remnant voltage of apixel undergoing said transition is determined prior to said transition,and the first or second waveform is used for said transition dependingupon the determined remnant voltage.